(a) Field of the Invention
The present invention relates to a focusing means for a zoom lens system which enables one to focus a zoom lens system by only one cam, regardless of the variation in the focal length of the zoom lens system as a whole, when focusing the zoom lens system by moving an arbitrary lens group constituting the zoom lens system. This application is a continuation-in-part of our prior application Ser. No. 271,644 filed June 8, 1981.
(b) Description of the Prior Art
Generally, known zoom lens systems are arranged to be focused by moving the foremost lens group thereof. However, in case of a wide-angle zoom lens system, whether or not its front lens group is positive or negative, vignetting in the marginal portion of the field angle caused when the zoom lens system is focused on an object at a comparatively short distance becomes very large, when the zoom lens system is focused by moving the foremost lens group only, unless the lens diameter of the foremost lens group is made very large. In case of a large zoom-ratio zoom lens system with a comparatively small field angle, it is sometimes so arranged that the foremost lens group is divided into several portions and focusing is carried out by moving one of those portions. In this case, however, the number of lenses constituting the lens system increases because the foremost lens group is divided into these several portions.
To eliminate the above-mentioned disadvantage, it may be considered that instead of focusing by moving the foremost lens group only, focusing of the lens system by moving a lens group other than the foremost lens group may be effected by moving a plurality of lens groups including or not including the foremost lens group, or by moving one or more lenses or lens components in those lens groups. In that case, however, the focusing plane moves according to the change in the focal length of the lens system caused by zooming, even when the lens system is focused on an object at the same distance. Therefore, the amount of movement of the lens component for focusing should be changed accordingly in relation to zooming. If one attempts to realize the abovementioned change in the amount of movement by using a cam mechanism, a complicated two-dimensional cam is required, and this is in fact impracticable.